It is likewise known from published German patent application document 10 2010 021 960 A1 to measure the time duration of a post-pulse oscillation in order to detect a blocked state of an ultrasonic sensor. However, in the related art, the ultrasonic sensor is classified as blocked if the duration of the post-pulse oscillation exceeds a predetermined limiting value. It may then be assumed that ice or dirt is adhering to the diaphragm, because the inertia of the diaphragm is thereby increased. To check the plausibility of a detected blockage with the aid of the ultrasonic sensor, it is also known from the related art to additionally measure an echo of the excitation pulse, and from that, to ascertain a distance value. If this distance value does not change even in response to a movement of the motor vehicle of its own, then highly likely, it is a case of an object adhering to the diaphragm.
Instead of the decay period, it is known from published German patent application document DE 10 2009 040 992 A1 to measure the decay frequency. If the diaphragm is iced or dirt is clinging to it, then the flexural stiffness of the diaphragm changes, resulting in an altered decay frequency.
The detection of a snow-covered diaphragm based on the exceedance of a decay time period is also described in published German patent application document DE 101 42 075 A1. To check the plausibility of a detected occlusion, the outside temperature is also monitored.
The German Patent 101 64 760 B4 describes, upon parking a motor vehicle and shutting off the engine, thus, upon ending vehicle operation, to store the last distance value that was measured by an ultrasonic sensor with respect to an object extraneous to the vehicle. If the motor vehicle is then started again, thus, vehicle operation is commenced again, a distance value is measured once more. By comparing the distance values, it is determined whether snow or slush has gotten onto the diaphragm of the ultrasonic sensor while the motor vehicle was parked.
In connection with a radar sensor, it is known from published German patent application document DE 10 2007 026 033 A1 to measure a distance value of a motor vehicle to an object located by the radar sensor, and upon a movement of the motor vehicle of its own, to classify the radar sensor as iced if, in spite of the vehicle movement, the distance value does not change over several measuring cycles. However, the motor vehicle must move with a minimum speed for a reliable detection.
The published German patent application document 10 2005 057 973 A1 describes a functional test for an ultrasonic sensor of a motor vehicle, in which such a powerful ultrasonic signal is emitted that under usual conditions, this ultrasonic signal would have to be reflected by ground in front of the motor vehicle and received again. If only a highly attenuated echo or no echo at all is received from the ground, an occluded or defective ultrasonic sensor is assumed.
The distance values to foreign objects in the surroundings of the motor vehicle ascertained by ultrasonic sensors may be entered into a digital surrounding-field map to permit ascertainment of the relative positions of the foreign objects, taking into account self-movements of the motor vehicle, even if the foreign objects no longer lie in the sensing range of the sensors. To that end, for example, a maneuvering method for vehicles is known from published German patent application document DE 10 2007 030 769 A1. A digital surrounding-field map of this kind is also described in published international patent application document WO 2009/119577 A1.
The described decay time of the diaphragm of an ultrasonic sensor may be defined as follows: The decay time may begin with the pulsing by the excitation pulse and may end if the maximum amplitude of a period of oscillation of the natural oscillation is less than a predetermined limiting value.
For instance, a decay time of a free, unblocked diaphragm may lie in a range of 1000 μs to 1400 μs. On the other hand, a diaphragm encumbered with ice or dirt may have a decay time greater than 2000 μs. The detection of an occlusion of the diaphragm based on a prolonged decay time has proven to be problematic for the case when an echo of a foreign object strikes the ultrasonic sensor precisely when the amplitude of the natural oscillation drops below the amplitude threshold value mentioned. In that case, the ending of the natural oscillation is not recognized, but rather the natural oscillation together with the arriving echo is measured as prolonged decay. For this case, however, the ultrasonic sensor must not be classified as blocked or occluded, but rather, a warning signal must be output for an obstacle located close to the motor vehicle. Otherwise, the system is not safe.
A further problem with the measurement of the decay time is that the decay time only changes if dirt or ice adheres directly to the diaphragm itself, so that its dynamic behavior changes. However, in the case of especially thick dirt or ice crusts, it has turned out that these crusts do not co-oscillate with the diaphragm, but instead, the diaphragm detaches from the crust and oscillates freely under it with only a slightly altered decay time. Thus, even given a thick encrustation, the occlusion of the ultrasonic sensor cannot be detected on the basis of the decay time.